The present invention relates to a blade for a turbine, e.g. aircraft engine, gas turbine, steam turbine, etc. More specifically, the present invention relates to the cooling of a turbine blade tip shroud. As a non-limiting example the invention and its background are described with reference to a gas turbine.
A gas turbine is typically comprised of a compressor section that produces compressed air. Fuel is mixed with a portion of the compressed air and burned in one or more combustors, thereby producing hot compressed gas. The hot compressed gas is expanded in a turbine section to produce rotating shaft power. The turbine section is typically comprised of a plurality of alternating rows of stationary vanes (nozzles) and rotating blades (buckets). Each of the rotating blades has an airfoil portion and a root portion by which it is affixed to a rotor.
Since the blades are exposed to the hot gas discharged from the combustors, cooling methods are required to obtain a useful design life cycle. Traditionally, blade cooling is accomplished by extracting a portion of the compressed air from the compressor and directing it to the turbine section, thereby bypassing the combustors. After introduction into the turbine section, this cooling air flows through passages formed in the airfoil portions of the blades. Often, radial passages are provided that discharge the cooling air radially outwardly at the blade tip.
On many rotating airfoils, integral tip shrouds are used on the radially outer end of the blade to create an outer surface of the passage through which the hot gases must pass. Having the shroud as a part of the airfoil results in an increase in performance for the engine. As such, it is desirable for the entire outer surface to be covered by the tip shrouds. However, integral shrouds on rotating airfoils are highly stressed parts due to the mechanical forces applied via the rotational speed. The high temperature environment coupled with the high stresses makes it a challenge to design a shroud that will effectively perform over the entire useful life of the remainder of the blade. Two methods for resolving this challenge are to reduce the stress and/or reduce the temperature.
A common method for reducing the stress is to remove a portion of the overhanging shroud (scallop the shroud), thus reducing the load applied. Reducing the tip shroud coverage, however, results in a detriment to engine performance. In addition or in the alternative, cooling air discharging at the blade tip flows over the radially outward facing surface of the shroud so as to provide a measure of film cooling.